The present invention generally relates to an implantable cardiac device. The present invention more particularly relates to an implantable pacemaker capable of detecting and providing therapy for repetitive non-reentrant ventriculo-atrial synchronous (RNRVAS) rhythms.
Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as a pacing system. The pacing system is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, which electrically couple the pacemaker to the heart.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient""s own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves)and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses the same chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
A popular mode of operation for dual-chamber pacemakers is the DDD mode. Specifically, DDD systems provide atrial pacing during atrial bradycardia, ventricle pacing during ventricular bradycardia, and atrial and ventricular pacing during combined atrial and ventricular bradycardia or heart block also known as AV block. In addition, DDD systems provide an atrial synchronous mode. This enables ventricular activity to track atrial activity to more closely approximate the normal response to exercise, or other physiological activity demanding a faster heart rate, by permitting a rate increase to occur commensurate with the rate of the sensed P waves. This advantageously increases cardiac output and facilitates maintenance of AV synchrony.
Many dual-chamber pacemakers further incorporate a physiologic sensor. Such sensors are employed to detect the patient""s degree of activity for regulating the heart rate. Hence, as the patient becomes more active, requiring increased cardiac output, the stimulation rate of the pacemaker is increased. When the patient becomes less active, requiring reduced cardiac output, the stimulation rate of the pacemaker is in turn decreased.
Dual-chamber pacemakers implement two main timing intervals to support their operation. These intervals are referred to as the AV delay interval and the VA interval, also referred to as the atrial escape interval. The AV delay interval is the time from an atrial pacing pulse or a detected P wave, if atrial activity is sensed, to when the next ventricular pacing pulse is to be delivered in the absence of a preoccurring R wave. Such pacing is referred to as atrial synchronous ventricular tracking and atrioventricular sequential pacing.
Similarly, the VA interval or atrial escape interval is the time from a detected R wave or a ventricular pacing pulse to when a next atrial pacing pulse is to be delivered absent a preoccurring P wave. This pacing is referred to as atrial demand pacing.
One condition that may occur during atrial synchronous pacing is a pacemaker mediated tachycardia (PMT). A PMT can result when the atrial sensing circuit detects a retrograde P wave (a P wave induced by a ventricular pacing pulse retrogradedly conducted from the ventricles back to the atria). When this occurs, the pacemaker initiates an AV delay interval and subsequently provides a ventricular pacing pulse at the end of the AV delay interval or the maximum tracking rate interval, which ever ends later. Repeated cycles of this stimulation pattern are sustained by the heart tissue retrograde conduction and by the pacemaker anterograde conduction.
Methods for preventing PMT are well known in the art. One such known method involves the use of a post ventricular atrial refractory period (PVARP) initiated by a ventricular pacing pulse or the detection of an intrinsic ventricular event which prohibits the atrial sensing circuit from sensing the retrograde P wave. The length of the PVARP is generally selected to be longer than the retrograde response time and may be divided into a first or absolute refractory period wherein no sensing is permitted, followed by a second or relative refractory period during which atrial sensing is permitted but the pacemaker is not permitted to respond to an atrial event sensed by the atrial sensing circuit. Hence, an atrial event sensed during the relative refractory period will not initiate an AV delay interval.
Unfortunately, the PVARP intended to prevent a PMT may lead to another rhythm referred to herein as a xe2x80x9crepetitive non-reentrant ventriculo-atrial synchronousxe2x80x9d (RNRVAS) rhythm. It is based on the ability of a patient""s heart to sustain retrograde conduction and the coincidence of timing intervals in both the patient and the pacemaker. As with a PMT, RNRVAS can be initiated by any phenomenon that would lead to AV disassociation and might trigger a PMT. A most common trigger mechanism is a premature ventricular contraction (PVC) which is generally defined as an R wave which occurs immediately succeeding a previous R wave or ventricular paced complex without an intervening P wave or atrial pacing pulse. A PMT is not initiated because the PVARP is programmed to a sufficient duration preventing the retrograde P wave from being tracked. However, if the pacing rate is high either due to a high programmed base rate or a physiologic sensor-driven rate being high, the atrial escape interval may time out and deliver an atrial pacing pulse at a time when the atrial myocardium is still physiologically refractory from the retrograde P wave. Hence, even though the atrial pacing output may be well above the atrial capture threshold, it will not capture the atria because it occurs at a time when the atrial myocardium is in its physiologic refractory period and cannot be depolarized. Hence, the RNRVAS rhythm results due to a combination of the high pacing rate and the non-detected retrograde P wave which causes the atrial tissue to not yet be recovered at a time when the atrial pacing pulse is delivered at the end of the atrial escape interval. A more detailed description of the manner in which the RNRVAS rhythm may be initiated follows subsequently in the detailed description of FIG. 3.
This rhythm is consistent with normal pacemaker function and represents a mismatch between the physiologic parameters of the patient""s heart with the parametric settings of the pacemaker. The RNRVAS rhythm can result in significant symptoms such as a significant decrease in both blood pressure and cardiac output, palpitations, dizziness and lightheadedness.
The present invention provides a system and method for detecting and treating a repetitive non-reentrant ventriculo-atrial synchronous (RNRVAS) cardiac rhythm. The system and method are adapted for use in an implantable cardiac stimulation device including a pulse generator that delivers atrial and ventricular pacing stimulation pulses to a heart. The device provides the ventricular pacing pulses on demand following an AV delay interval after at least an atrial pacing pulse and the atrial pacing pulse follows an atrial escape interval after a natural or paced ventricular event. In accordance with a broader aspect of the present invention, when an RNRVAS rhythm is recognized and detected, a therapy control circuit lengthens the atrial escape interval for at least a next cardiac cycle. This allows more time for the atrial myocardium to recover on a physiologic basis so that when an atrial pacing pulse is delivered by the pulse generator at the end of the extended atrial escape interval, the atrial pacing pulse will be able to capture the atria.
Preferably, as the atrial escape interval is increased, the AV delay interval is decreased by the same amount. This maintains the present ventricular stimulation rate of the heart.
Following the next cardiac cycle, the atrial escape interval may be shortened and the AV delay interval increased to programmed or rate dependent values. This permits the system to determine if the applied therapy was successful in terminating the RNRVAS rhythm.
In accordance with further aspects of the present invention, when an RNRVAS rhythm is detected, the therapy control circuit may increase the atrial escape interval for a predetermined number of cardiac cycles. If the RNRVAS rhythm thereafter persists, the system may pause for a preset time or until the cardiac rate falls below a predetermined rate. After the pause, the system is once again ready to detect and treat an RNRVAS rhythm. This further aspect of the present invention may be particularly beneficial for patients suffering from high grade AV block and who exhibit retrograde conduction.
To detect an RNRVAS rhythm, the system may register an atrial-ventricular complex type for each cardiac cycle. The complex types preferably include AR complexes wherein atrial pacing pulses are followed by R waves and AV complexes wherein atrial pacing pulses are followed by ventricular pacing pulses. A change from an AR complex to an AV complex may denote a loss of atrial capture and hence may be used to detect an RNRVAS rhythm. A further criteria may include the condition that the cardiac rate be higher than a predetermined rate in addition to the AR complex to AV complex change.
The complex discrimination may be achieved by the system noting the delivery of atrial and ventricular pacing pulses and the detection of R waves. Alternatively, the complex discrimination may be achieved through morphology detection capable of discerning a fully inhibited morphology and a fully paced morphology. Upon RNRVAS rhythm redetection, a morphology other than a fully paced morphology or a fully inhibited morphology would indicate a fusion beat and atrial capture with subsequent intact AV nodal conduction.
For patients with high degree AV block and who are therefore paced continually with AV complexes, the detection criteria may alternatively be an increase in cardiac rate to above a predetermined rate.
Still further, RNRVAS rhythm detection may be achieved by morphology detection of atrial evoked responses, atrial loss of capture, and atrial fusion beats. Here, if an atrial loss of capture determined through morphology detection is preceded by a ventricular pacing pulse, the RNRVAS rhythm may be declared and appropriate therapy initiated.
Still further, an RNRVAS rhythm may be detected by sensing atrial activity during the post ventricular atrial refractory period. A retrograde P wave occurring during this time will render the atria refractory requiring therapy to provide an atrial pacing pulse at a time when the atria are fully recovered. A further condition to this manner of detection may be the delivery of AV pacing complexes at a rate above a predetermined rate.